1. Field of the Invention
The present invention relates to a power supply device for driving an amplifier, and more particularly, to a power supply device capable of reducing power consumption of an amplifier and enlarging an output range of the amplifier.
2. Description of the Prior Art
An amplifier is an electronic device used in a variety of electronic products, which can amplify signal amplitudes. The amplifier appears in many types and structures, which can be simply composed of a bipolar junction transistor (BJT) or a metal oxide semiconductor (MOS) transistor, or be integrated by complicated circuits such as OP 741.
The amplifier is widely used in many applications. For example, U.S. Pat. No. 7,061,327 discloses a headset using a single voltage supply to drive an operational amplifier of the headset via a charge pump. Furthermore, U.S. Pat. No. 5,289,137 discloses an integrated circuit using a single power supply and a charge pump for driving an operational amplifier. Both of the U.S. Pat. No. 7,061,327 and No. 5,289,137 use single power supplies with charge pumps for driving the amplifier. However, shortcomings of these two patents are power consumption and restriction of output ranges. The details are described as bellows. Please refer to FIG. 1, which reveals a schematic diagram of signals corresponding to an amplifier in FIG. 4 of the U.S. Pat. No. 7,061,327. Voltages VDD and VSS respectively represent a positive voltage and a negative voltage for driving the amplifier. Since the U.S. Pat. No. 7,061,327 uses single power supply with a charge pump, the voltage VDD can keep a relation by VSS=−VDD. In addition, the charge pump is substantially composed of capacitors and thereby, generates the Voltage VSS via charging and discharging of the capacitors. Under this circumstance, the voltage VSS performs as an unstable linear curve rippling as the capacitors are charged and discharged. Since amplitudes of output signals of the amplifier must be restricted between the voltages VDD and VSS, negative amplitude of the amplifier may be curtailed as a variation range of the voltage VSS becomes large. In order to solve the problem, the prior art can increase the voltage VDD so that the output range of the amplifier can be enlarged, or increase capacitance of the capacitors in the charge pump. However, increasing the voltage VDD represents increasing power consumption, while increasing capacitance of the capacitors in the charge pump represents increasing the capacitor size.
Therefore, using a single power supply with a charge pump for driving an amplifier results in a restriction of an output range of the amplifier, and power consumption cannot be efficiently reduced. Besides, if a single power supply is used for driving multiple amplifiers, current variation produced by switching transistors of a certain amplifier may cause malfunction of the power supply, and reduces system efficiency.